A typical computer system arrangement includes a circuit board on which are mounted various components of the system, such as the central processing unit (CPU), main memory, chipset controllers, and certain peripheral devices. The components are laid out on various parts of the circuit board, with interconnecting electrical wires (that make up buses and other links) enabling communications among the components.
Two types of signals can be routed over the interconnecting electrical wires: single-ended signals and differential signals. A differential signal is the difference between two signals routed over two different wires that are connected to a differential receiver. On the other hand, a single-ended signal is routed over one wire.
In many computer systems, some single-ended signals are connected to differential receivers. In such an arrangement, one input of each differential receiver is connected to a single-ended signal, while the other input of the differential receiver is connected to a reference voltage signal.
Single-ended signal communications are usually more sensitive to noise within the computer system than differential signal communications. With differential signal communications, noise usually affects the two wires that carry the signals making up the differential signal equally. As a result, the noise effect on the two wires is cancelled out. However, because only one electrical wire is used to carry a single-ended signal, the noise cancellation effect is not available for the single-ended signal connected to one input of a differential amplifier.
A differential receiver that receives a single-ended signal is able to function properly (accurately detect the state of the single-ended signal) over a range of voltage levels of the reference voltage signal. The range of voltage levels of the reference voltage signal over which receivers connected to single-ended signals continue to work properly defines the margin of the reference voltage signal. It is desired that the margin be as large as possible to reduce likelihood of errors caused by noise or other factors during computer operation. The wider the margin of the reference voltage signal, the better the quality of a single-ended signal communications link.
Conventionally, during the design process of a given computer model, a prototype of the computer model (including a prototype circuit board) is built. Typically, a potentiometer is provided on the prototype circuit board to adjust a voltage level of a reference voltage signal for differential receivers connected to single-ended signals. The potentiometer is typically connected to a circuit that generates the reference voltage signal, with the potentiometer used to adjust the reference voltage up and down until operation of a single-ended signal communications link fails.
Although the testing of single-ended signal communications links in a prototype computer system provides some indication of the performance of single-ended signal receivers, the behavior of prototype computer systems may differ from actual production computer systems (computer systems manufactured for mass sale). One of the reasons for the different behavior is the possibility of different arrangements of components, such as CPUs, memory, and peripheral devices, in the computer systems. One computer model may have several different configurations with different arrangements and types of components.
For example, some peripheral devices are optional devices that may not be present in all configurations of the computer model. Also, CPU speeds and memory sizes and speeds may vary in different configurations. In the different configurations of the computer model, different noise levels may be present. The different noise levels present in the different configurations may affect the reference voltage margin in different ways.
Other factors may also cause the reference voltage margin to vary. Such other factors include the drive strengths of signal buffers, data or clock jitter, receiver sensitivity, and transmission line loss. Thus, the determination of the reference voltage margin in the prototype computer system may not provide an accurate indication of the reference voltage margins present in production computer systems.